It is known when validating coins to perform two or more separate tests on the coin, and to determine that the coin is an authentic coin of a specific type or denomination only if all the test results equal or come close to the results expected for a coin of that type. For example, some known validators have inductive coils which generate electromagnetic fields. By determining the influence of a coin on those fields the circuit is capable of deriving different measurements which are predominantly determined by the thickness, the diameter and the material content of the coins. A coin is deemed authentic only if all three measurements indicate a coin of the same type.
This is represented graphically in FIG. 1, in which the three orthogonal axes P.sub.1, P.sub.2 and P.sub.3 represent the three independent measurements. For a coin of type A, the measurement P.sub.1 is expected to fall within a range (or window) W.sub.A1, which lies within the upper and lower limits U.sub.A1 and L.sub.A1. Similarly the properties P.sub.2 and P.sub.3 are expected to lie within the ranges W.sub.A2 and W.sub.A3, respectively. If all three measurements lie within the respective windows, the coin is deemed to be an acceptable coin of type A. In these circumstances, the measurements will lie within an acceptance region indicated at R.sub.A in FIG. 1.
In FIG. 1, the acceptance region R.sub.A is three dimensional, but of course it may be two dimensional or may have more than three dimensions depending upon the number of independent measurements made on the coin.
Clearly, a coin validator which is arranged to validate more than one type of coin would have different acceptance regions R.sub.B, R.sub.C, etc., for different coin types B, C, etc.
In the prior art, each acceptance window is always predetermined before the test is carried out. Some validators have means for adjusting the acceptance windows. The purpose of the adjustment is either to increase the proportion of valid coins which are determined to be acceptable (by increasing the size of the acceptance window) or to reduce the number of non-genuine coins which are erroneously deemed to be valid (by reducing the size of the acceptance window). Adjustment of the window is carried out either manually, or automatically (e.g. as in EP-A-0155126). In any event, the result of the window adjustment is that the upper and lower limits of the acceptance window are predetermined.
However, by reducing the acceptance windows in order to avoid accepting non-genuine coins, it is possible that genuine coins will then be found to be invalid. Conversely, by increasing the acceptance windows to ensure that a maximum number of genuine coins are found to be valid, more non-genuine coins may also be determined to be valid. The consequence is that adjustment of windows may have adverse effects as well as beneficial effects, and may not increase the "acceptance ratio" (i.e. the ratio of the percentage of genuine coins accepted to the percentage of non-genuine coins accepted), or may only increase this ratio by a small amount.
In the field of banknote validation, measurements are also compared with acceptance regions generally of the form shown in FIG. 1. Similar problems thus arise when modifying the acceptance windows to try to avoid accepting non-genuine notes or rejecting genuine notes.
International Patent Application No. PCT/GB90/01588 and Irish Patent Application No. 3708/90 propose a method of validating items of money comprising deriving at least two different measurements of a tested item, determining whether each measurement lies within a respective range associated with a particular money type, and producing a signal indicating that money of that type has been tested if all measurements fall within the respective ranges for that type, wherein the respective range for at least one of the measurements varies in dependence on at least one other measurement.
The reference to "different measurements" is intended to indicate the measurement of different physical characteristics of the tested item, as distinct from merely taking the same measurement at different times to indicate a single physical characteristic or combination of such characteristics. For example, in GB-A-1 405 937, and in several other prior art arrangements, the time taken for a coin to travel between two points is measured. Although this could be regarded as taking two time measurements and determining the difference, the purpose is simply to obtain a single measurement determined by a particular combination of physical characteristics, and therefore this does not represent "different measurements" as this is understood in the present case. Similarly, it is known to take two successive measurements dependent on the position of a coin with respect to a sensor as the coin passes the sensor, and then to take the difference between those two measurements. Again, this difference would represent a single measurement determined by a single combination of physical characteristics (e.g. a variation in the surface contour of the coin).
In many circumstances, using this technique results in an improved acceptance ratio. For example, it may be found empirically that measurements P.sub.1 and P.sub.2 of valid money items of type A tend to lie within ranges W.sub.A1 and W.sub.A2 respectively. However, it may also be found empirically that genuine items having a large value P.sub.1 are unlikely also to have a large value P.sub.2. Using the techniques mentioned above, the upper limit of range W.sub.A2 can be made smaller when large values of P.sub.1 are detected. This would not significantly affect the number of valid items which are erroneously rejected, but would cause counterfeit items which may have large values of P.sub.1 and P.sub.2 to be rejected.